Most conventional automatic gain control (AGC) circuits in optical navigation products use image statistics from the image that is currently being processed for navigation. Typically, the minimum pixel value and the maximum pixel value along with the average pixel value of the image sensor array for the current image are observed. Using these observed values, the exposure time and/or the illumination drive are then adjusted, if needed, to improve the next image.
The above AGC approach works well in optical computer mice and other imaging devices in which the image frames are acquired much quicker than the changes in the surface/scene illumination level. However, this AGC approach does not work well for imaging devices in which the image-to-image illumination level changes by a significant amount because the anticipated illumination level could be incorrect. In such instances, the image information may not be optimized, or worse yet, may be lost. In optical computer mice, this means that the tracking will degrade or fail all together.
In optical navigation products, the delay between the images was kept small enough to ensure only a small amount of the image could change, and the anticipated exposure would be sufficiently close to allow the navigation sensor to accurately track the surface changes. This requires the frame rate to be high, and results in a high number of images being acquired and processed, which in turn causes energy to be expended. In portable devices, this limits the battery life that can be achieved. In conventional optical computer mice, battery life was conserved by monitoring the amount of time the computer mouse was inactive, and the frame rate was gradually reduced as the period of inactivity increased. These Reset/Sleep modes would ratchet down the number of frames per second (FPS) to save power. When the mouse was moved, the highest frame rate would then be executed and the AGC would be adjusted until an acceptable image exposure was reached. Only when the acceptable image exposure was reached would tracking begin.
Although the above power saving technique works well for its purpose, there is a need for a system and method for providing ACG in an imaging device that can further reduce the amount of power being expended during operation.